Optical Coherence Tomography (OCT) is a technique for performing high-resolution cross-sectional imaging that can provide images of tissue structure on the micron scale in real time (Huang et al. “Optical Coherence Tomography” Science 254(5035):1178 1991). OCT is a method of interferometry that determines the scattering profile of a sample along the OCT beam. Each scattering profile is called an axial scan, or A-scan. Cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse locations on the sample. OCT provides a mechanism for micrometer resolution measurements.
It has been demonstrated that frequency-domain OCT (FD-OCT) has advantages over the original time-domain OCT (TD-OCT) (see for example R. Leitgeb, C. K. Hitzenberger, and A. F. Fercher, “Performance of Fourier domain vs time domain optical coherence tomography,” Optics Express, 11, pp 889-894 (2003)). FD-OCT methods use the fact that interference between light scattered from the sample and the reference beams causes spectral interference fringes, or modulation in the intensity of the combined beam as a function of optical frequency. The spacing of the interference fringes depends on the difference in optical group delay between the light scattered from the sample and the reference light. In FD-OCT the optical path length difference between the sample and reference arms is not mechanically scanned. A full A-scan can be obtained in parallel for all points along the sample axial line within a short time, typically determined by the wavelength sweep rate of a swept source in swept-source OCT (SS-OCT) or the line scan rate of the line scan camera in spectral-domain OCT (SD-OCT). In SD-OCT, a grating, a prism, or other means is used to disperse the output of the interferometer into its optical frequency components. The intensities of these separated components are measured using an array of optical detectors, each detector receiving an optical frequency or a fractional range of optical frequencies. The set of measurements from these optical detectors forms an interference spectrum. Typically the light source emits a broad range of optical frequencies simultaneously. In SS-OCT, a laser source is rapidly tuned through a range of wavelengths encoding the spectral information in time.
While FD-OCT (SD-OCT and SS-OCT) has already demonstrated tremendous improvement in acquisition speed over time-domain OCT, there are still many advantages of further increasing the speed of OCT systems. Many applications can benefit from denser sampling, resulting in finer details and higher resolution. At the same time, faster acquisition also helps to reduce motion artifacts. In ophthalmic OCT, increased data acquisition rate could be used to acquire higher density and wider field of view (FOV) cube data. Multi-frame averaging could result in speckle suppression and improved clarity in visualizing layered structure of the retina. Functional extensions of OCT such as OCT angiography could also benefit from high acquisition speeds as these techniques typically require repeated measurements and are susceptible to motion of the eye.
However, an increase in speed comes at the cost of sensitivity of the OCT system, if the optical power on the sample remains unchanged. In ophthalmic OCT systems, the incident power on the eye cannot exceed safe exposure limits as determined by laser and safety standards. Hence, while faster acquisition systems can obtain denser data sets, or make measurements more immune to motion artifacts, the sensitivity decreases. Therefore, high-sensitivity in vivo imaging is more challenging at higher speeds. High-sensitivity, however, is critical for several OCT imaging applications, for example, visualization of structures with typically low SNR including but not limited to the vitreous and the choroid. One commercial ophthalmic OCT system from Nidek, the RS-3000 Advance, advertises a user selectable OCT sensitivity option in which the system can acquire images in one of three sensitivities: ultra fine, fine, and regular trading off scan speed and sensitivity based on ocular pathology (http://www.nidek-intl.com/products/diagnosis/rs-3000advance.html).
Several research studies have looked into exploring the performance of OCT systems at various acquisition speeds. Benjamin Potsaid et al. explored the trade-offs between system acquisition speed and sensitivity performance of the system for ophthalmic imaging (B. Potsaid et al., “Ultrahigh Speed Spectral/Fourier domain OCT Ophthalmic Imaging at 70,000 to 312,500 axial scans per second,” Optics Express, 16 (19), pp 15149-15169 (2008)). Potsaid et al. built four different OCT system configurations each operating at a different speed. Applying this approach to commercial systems would greatly impact system size and cost. In a later study, Potsaid et al. demonstrated a MEMS tunable VCSEL light source with sweep rates changing from 60 kHz to 1 MHz (B. Potsaid et al., “MEMS tunable VCSEL light source for ultrahigh speed 60 kHz-1 MHz axial scan rate and long range centimeter class OCT imaging,” SPIE Proceedings, 8213, 82130M-1 (2012)). However, this is not a preferred solution as it is challenging to maintain similar spectral characteristics of the light source at different speeds, and special design or calibration efforts are needed. In this design, a booster semiconductor optical amplifier was used to approximately maintain similar bandwidth at different speeds. However, such a solution would add to the cost of the laser.
Johnson at al. also proposed multi-speed operation using an SS-OCT system (WO Publication 2013/154953). However, their approach requires a swept-source with multiple sweep speeds and a corresponding k-clock interferometer for each sweep speed for optimal use of detection bandwidth. Adding multiple k-clock interferometer options and synchronization increases the complexity of the system. And finally, this solution also requires a flexible rate swept source that may add complexity to the laser design. Everett et al. proposed the method to increase the A-scan rate of the system and have the option of multiple speeds, but the higher A-scan rate is achieved at the cost of reduction of axial resolution (U.S. Pat. No. 8,500,279).
To summarize, the solutions described in the prior art for multiple speed OCT operations add complexity or cost to the system. Specifications for detector electronics such as detection bandwidth, interferometer design to optimize reference power, and sweep rate of the laser are closely interlinked parameters that cannot be independently adjusted without impacting the optimized performance of the SS-OCT system. For example, for a given OCT imaging depth and unchanged axial resolution, the detection bandwidth will need to increase proportionally with an increase in the swept laser speed. In addition, reference power needs to be optimized for a given detector system to minimize the relative contribution of detector noise.
In SD-OCT systems, the reference optical power levels are typically optimized for reducing the noise contribution due to camera noise and autocorrelation artifacts. By increasing the reference power, the contribution due to detector noise is reduced relative to the photon shot noise, and the detection is described as shot noise dominated. While higher reference power is desired, it should not exceed the Full-Well Capacity (FWC) of the camera. In an SD-OCT system with a fixed reference power, if the camera exposure time is adjusted for various speeds, then the reference power would need to be optimized for the slowest operating speed to avoid camera saturation issue. This will lead to sub-optimal sensitivity performance at higher acquisition speeds. Alternatively, the sensitivity of multi-speed SD-OCT systems can be optimized by adjusting the reference arm power for different acquisition speeds to maintain fixed exposure energy on the camera per acquisition. However, such a design would increase the cost and complexity of the instrument.
In light of the above limitations, it would therefore be desirable for a single FD-OCT system (either SD-OCT or SS-OCT), to operate at multiple acquisition speeds with optimum sensitivity performance detection with minimal added complexity and system adjustments.